\begin{table}[tbp] \centering
\newcolumntype{C}{>{\centering\arraybackslash}X}

\begin{tabularx}{\linewidth}{lCCCC}

\toprule
{Mediation variable}&{War bonds}&{Volunteers}&{Medals}&{PCA} \tabularnewline
\midrule \addlinespace[\belowrulesep]
1940 employment&6.0\%&--8.7\%&2.9\%&1.3\% \tabularnewline
1939 wage&2.1\%&--0.3\%&--0.2\%&--0.4\% \tabularnewline
1940 FDR vote share&0.5\%&2.0\%&--2.2\%&0.1\% \tabularnewline
\bottomrule 

\end{tabularx}
\end{table}
